A tensioning device, such as a hydraulic tensioner, is used as a control device for a power transmission chain, or similar power transmission device, as the chain travels between a plurality of sprockets. In this device, the chain transmits power from a driving shaft to a driven shaft, so that part of the chain is slack and part of the chain is tight. Generally, it is important to impart and maintain a certain degree of tension in the chain to prevent noise, slippage, or the unmeshing of teeth in the case of a toothed chain.
Prevention of such slippage is particularly important in the case of a chain driven camshaft in an internal combustion engine because jumping of teeth will throw off the camshaft timing, possibly causing damage or rendering the engine inoperative. However, in the harsh environment of an internal combustion engine, various factors can cause fluctuations in the chain tension.
For instance, wide variations in temperature and thermal expansion coefficients among the various parts of the engine can cause the chain tension to vary between excessively high or low levels. During prolonged use, wear to the components of the power transmission system can cause a decrease in chain tension. In addition, camshaft and crankshaft induced torsional vibrations cause considerable variations in chain tensions. Reverse rotation of an engine, occurring for example in stopping or in failed attempts at starting, can also cause fluctuations in chain tension. For these reasons, a mechanism is desired to remove excessive tensioning forces on the tight side of the chain and to ensure the necessary tension on the slack side of the chain.
Hydraulic tensioners are a common method of maintaining proper chain tension. In general, these mechanisms employ a lever arm that pushes against the chain on the slack side of the power transmission system. This lever arm must push toward the chain, tightening the chain, when the chain is slack, and must be very rigid when the chain tightens.
To accomplish this result, a hydraulic tensioner typically comprises a rod or cylinder as a piston, which is biased in the direction of the chain by a tensioner spring. The piston is housed within a cylindrical piston housing, having an interior space which is open at the end facing the chain and closed at the other end. The interior space of the piston housing contains a pressure chamber in connection with a reservoir or exterior source of hydraulic fluid. The pressure chamber is typically formed between the piston housing and the piston, and it expands or contracts when the piston moves within the piston housing.
Typically, valves are employed to regulate the flow of fluid into and out of the pressure chamber. For instance, an inlet check valve typically includes a ball-check valve that opens to permit fluid flow in to the pressure chamber when the pressure inside the chamber has decreased as a result of outward movement of the piston. When the pressure in the pressure chamber is high, the inlet check valve closes, preventing fluid from exiting the pressure chamber, which in turn prevents the piston chamber from contracting, which in turn prevents the piston from retracting, achieving a so-called "no-return" function.
In some tensioners, a pressure relief valve allows the tensioner to retract in response to large increases in chain tension. The pressure relief valve typically includes a spring biased check valve. This valve opens when the pressure in the pressure chamber becomes high. The opening of the pressure relief valve allows fluid to exit the pressure chamber, which allows the pressure chamber to contract, thus allowing retraction of the piston.
In operation, the tensioner spring and hydraulic fluid urge the piston outwards, balanced against the inward force of the chain. As the tension in the chain increases, the chain exerts a force on the piston in the direction of piston retraction. As the piston is forced in the retraction direction, the fluid pressure in the pressure chamber increases, but the inlet check valve prevents the fluid from exiting the pressure chamber. If the pressure increases to a predetermined level, the pressure relief valve opens, allowing the fluid to exit the pressure chamber.
If, due to chain wear or other factors, the tension in the chain reduces so that the force of the chain on the piston no longer balances the combined force of the hydraulic pressure and the tensioner spring, then the piston will extend toward the chain, thereby tensioning the chain. As the piston extends toward the chain, the inlet valve opens, allowing hydraulic fluid from the external source of fluid to fill the pressure chamber.
A number of challenges exist in the design of hydraulic tensioners, including high cost and difficulty of manufacture and assembly. Traditionally, hydraulic tensioners have been constructed of cast iron housing bodies with turned and centerless ground pistons. This type of construction is expensive and difficult to manufacture. A need exists for a lower cost hydraulic tensioner which is easier to manufacture and assemble.
One example of a tensioner design directed to reduced cost is described in Ojima et al., U.S. Pat. No. 5,037,357. Ojima et al. disclose a spring loaded tensioner including a body having a bearing surface, a first spring seated against the bearing surface and biasing a piston in a protruding direction. A second spring functions as a damper allowing the piston to retract in response to increasing tension in the belt or chain. The body may be made of sheet metal, allowing for low cost manufacturing. The disadvantages of this design include the reliance on springs to provide the "no return" and pressure relief functions. As a result, this design does not provide the advantages in performance provided by a hydraulic tensioner.
The present invention is directed to a lower cost hydraulic tensioner. A housing body has a bore, and a bore cup received in the bore. The bore cup may be deep drawn of a suitable metal. A piston is slidably received in the bore cup, forming a pressurized fluid chamber with the inside of the bore cup. The piston may also be constructed of drawn metal. Preferably, the fluid chamber is connected to the external source of fluid by a single passage. A valve system is assembled within a valve housing which is positioned within the piston. The valve system performs the inlet check function. In one embodiment, the valve system includes an integral inlet and pressure relief valve which performs both the inlet check function and the pressure relief function.
Because the pressure chamber is formed with the bore cup, rather than the housing body as in conventional tensioners, the housing body of the present invention can be constructed of materials, such as aluminum or plastic, which are less expensive than the cast iron which is employed by conventional tensioners. In addition, the positioning of the valve system within the bore cup eliminates the need to provide a valve system within the housing body. Thus, the positioning of the valve system within the bore also reduces the cost of the tensioner.
Furthermore, the design, employing a single passage connecting the pressure chamber with the external source of fluid, in contrast to a design having two or more passages connecting to the fluid chamber, also simplifies the design of the housing body, allowing for a lower cost hydraulic tensioner. In operation, the single passage design also reduces the amount of pressure fluid lost to the system, resulting in a more efficient and less costly tensioner system.
In some embodiments, the inlet check valve assembly employs the aperture in the bore cup as a valve seat. This design provides for a lower cost valve assembly compared to designs employing a separate member as a valve seat.
Accordingly, it is an object of the present invention to provide a hydraulic tensioner capable of improved response to fluctuations in chain tension. Another object of this invention is to provide a hydraulic tensioner for which the cost of manufacture is lower than the cost of conventional hydraulic tensioners. Another object of this invention is to provide a hydraulic tensioner having a housing body constructed of an inexpensive material. A further object of the invention is to provide a hydraulic tensioner having a bore cup, piston and/or a valve housing constructed of drawn metal. Still another object of this invention is to provide a hydraulic tensioner allowing for ease of manufacture and assembly.